Sera from over 500 patients with breast disease were analyzed for antibody to a breast cancer antigen by immunodiffusion. By this method, 9% of the patients with the diagnosis of fibroadenoma, 5% of the patients with fibrocystic disease and 16% of the patients with cancer of the breast had detectable antibody. Interestingly, only 0.5% of 918 screenees of the Detection Center for Breast Diseases had demonstrable antibody. Sera from patients with various diseases known to stimulate serum antibody failed to react with the breast cancer antigen in immunodiffusion. In addition, appropriate absorptions showed that the antibody was not directed to the "T" antigen or Forssman's antigens. The histopathologic studies demonstrated that the presence of antibody correlated well with lymphoid infiltration of the tumor and sinus histiocytosis of the axillary lymph nodes. Finally, survival data showed that of 22 patients with demonstrable antibody, none have died, with five out of five alive at three years. In contrast, in patients with breast cancer metastatic to the axillary lymph nodes and no demonstrable antibody, only 35% were alive at two years and only one out of five at three years.